Yes, as stated above, you will need to show proof of insurance in order for the CBSA to stamp your passport with a (IEC) work visa. If you don't have the insurance and they ask for it, you'll be let in (being from the UK), but only with a regular visitor visa - you'll then have to make a dash across the USA border to re-enter once you have the proper insurance.
Irish and Australian nationals can get an IEC work visa for up to 2 years, all other countries are limited to one year (some of which are renewable for another year). The trick for the Irish and Australians is buying a policy for the full two years. If you only have one year of insurance, they'll only issue the IEC work visa for that length of time. So, the travelance policy above is good in that they can issue for 18 months, but if you want two years worth, you'll need to look elsewhere.
BestQuote Travel Insurance has the widest selection of Canadian insurance companies to choose from, including policies for up to 2 years (there aren't alot of options for 2 years worth of insurance as the Canadian insurance companies haven't chased after the IEC work permit market (yet) - perhaps as more countries start to allow 2 year visas the insurance companies will get on board.
Most IEC visa holders end up finding employment that allows them to qualify for provincial health care. In Alberta, you can go on provincial health care (you become eligible for the benefits) as soon as you are eligible and apply (there's no three month waiting period for benefits to kick in, like there is in BC, ON, QC, or NB). So if you are buying a policy that allows you to get a pro-rated refund once you go on provincial health care - then you can cancel the policy after only a short time has passed. Technically, this is breaking the rules of the IEC program - because provincial health care doesn't provide repatriation benefits. But provincial health care in Canada is all incompassing, unlike visitors to canada insurance (travel health insurance) which has a lot of restrictions and limitations.
Lots of IEC visa holders tend to break this rule (and save themselves a few $100), once they have a provincial healthcare card to show the insurance company. Until the insurance companies put in any measures to stop this (change the wording of their refund policy), it will continue.
A word of caution: some companies won't let you cancel a policy when you go on provincial health care. So if this is something you may plan on doing, read the policy you are buying first.
You can also find useful info at:
https://www.bestquotetravelinsurance.ca/International-Experience-Canada-%28IEC%29-Insurance